1. Technical Field
The present invention relates to a test apparatus that tests a memory. More particularly, the present invention relates to a test apparatus that stores information concerning defective cells acquired as a result of a test.
2. Related Art
In manufacturing of a semiconductor device, it is essential to use a design (DFM: Design for Manufacturing) that optimizes the efficiency of the manufacturing process in order to enhance yield and decrease cost. For example, when a new semiconductor device manufacturing process is implemented, there are cases where sufficient yield cannot be achieved at a preliminary stage. In such a case, the cause of the insufficient yield is investigated and then a design of a mask pattern of an exposure apparatus is changed or an arrangement of the semiconductor device on a wafer is changed. Such improvements may be required more than once, which can result in the process of investigating the cause of the problem and solving the problem being repeated several times.
Japanese Patent Application Publication No. 1998-125092 is cited as an example of technology that involves testing a flash memory serving as the semiconductor device.
To increase the efficiency of the aforementioned process and begin efficient manufacturing quickly, it is desirable that the investigation of the cause of the problem be made more efficient. To facilitate the investigation of the cause of the problem, conventional test apparatuses for semiconductor devices record the problem arising in the semiconductor device for every memory cell and provide this information to a designer. In a preliminary stage of implementing the manufacturing process, however, there are cases where a block in which the problem arises in only some of the cells and a block in which the problem arises in many of the cells are mixed together in the same semiconductor device. In the block in which the problem arises in only some of the cells, knowing the position of the cells in which the problem arises is helpful in the investigation of the cause of the problem. On the other hand, in the block in which the problem arises in many of the cells, knowing the position of the defective cells is not helpful in investigating the cause of the problem because there are cases where a phenomenon affecting the entire block is the cause of the problem.
Furthermore, because the information concerning the problem of every cell is a large amount of data, it is desirable that the information be compressed and saved. When the cells in which the problem arises are recorded in order of the addresses, however, it is common for the compression ratio of the data compression to decrease when the locations at which the problem arises are many and spread out. In other words, not only is a block with too many problems unhelpful in the investigation of the cause of the problem, but there is also a concern that the storage capacity will be needlessly filled when the information concerning the problem is stored. Therefore, recording the problem of every cell for all the blocks in a uniform manner as described above is not efficient.